Archibald McArthur Johnston, assayer and metallurgist, was the son of William Johnston and his wife Agnes. He obtained the degree Master of Arts (MA) and became a member of the (British) Institution of Mining and Metallurgy. In 1894 he arrived in South Africa and four years later became a member of the Chemical and Metallurgical Society of South Africa (from 1902 the Chemical, Metallurgical and Mining Society of South Africa). In May 1899 he read a paper before the society on "An experiment in cyanide poisoning". It generated quite some discussion and was published in the society's Proceedings for 1897-1899 (Vol. 2, pp. 676-683; discussion pp. 722-723, 737-738, 766-767, 785-786). From 1906 he served as a member of the society's council, was joint vice-president for 1908/9, and president for 1909/10. His presidential address dealt with "The crystallization of iron and steel". Meanwhile he had read a note on "Gold crystals" in December 1908, which was published in the society's Journal (Vol. 9, p. 182). In 1905 he was an assayer to the Mount Morgan Gold Mining Company at Barberton. In 1912 he worked for Consolidated Gold Fields of South Africa and carried out dust determinations in mines of the Germiston district. That same year he read a paper on "Dust determination by filtration through sugar" (Ibid, 1911/2, Vol. 12, pp. 442-443), which facilitated the introduction of regular dust determinations in the mines as part of the battle against miners' phthisis. During the next two years he contributed a paper on "Investigation of magnetically separated iron from mill pulp" (Ibid, 1912/3, Vol. 13, pp. 50-54) and a note on "Fine screening" (Ibid, 1913/4, Vol. 14, p. 394). Also in 1912 he was one of the contributors to A textbook of Rand metallurgical practice, by R.S.G. Stokes* and others.
In 1940 Johnston was still listed as one of the society's surviving past presidents. He was married to Christina Barr Johnston, born Stewart, with whom he had two surviving sons.